Sulfate Metabolism in C4Flaveria Species Is Controlled by the Root and Connected to Serine Biosynthesis

The evolution of C 4 photosynthesis led to an increase in carbon assimilation rates and plant growth compared to C 3 photosynthetic plants. This enhanced plant growth, in turn, affects the requirement for soil-derived mineral nutrients. However, mineral plant nutrition has scarcely been considered in connection with C 4 photosynthesis. Sulfur is crucial for plant growth and development, and preliminary studies in the genus Flaveria suggested metabolic differences in sulfate assimilation along the C 4 evolutionary trajectory. Here, we show that in controlled conditions, foliar accumulation of the reduced sulfur compounds Cys and glutathione (GSH) increased with progressing establishment of the C 4 photosynthetic cycle in different Flaveria species. An enhanced demand for reduced sulfur in C 4 Flaveria species is reflected in high rates of [ 35 S]sulfate incorporation into GSH upon sulfate deprivation and increased GSH turnover as a reaction to the inhibition of GSH synthesis. Expression analyses indicate that the γ-glutamyl cycle is crucial for the recycling of GSH in C 4 species. Sulfate reduction and GSH synthesis seems to be preferentially localized in the roots of C 4 species, which might be linked to its colocalization with the phosphorylated pathway of Ser biosynthesis. Interspecies grafting experiments of F. robusta (C 3 ) and F. bidentis (C 4 ) revealed that the root system primarily controls sulfate acquisition, GSH synthesis, and sulfate and metabolite allocation in C 3 and C 4 plants. This study thus shows that evolution of C 4 photosynthesis resulted in a wide range of adaptations of sulfur metabolism and points out the need for broader studies on importance of mineral nutrition for C 4 plants.